In vitro, electrophysiology culture systems having microelectrode arrays (MEAs) can provide important insights into networks of electrically active cells. MEA-based electrophysiology culture systems can be configured to concurrently monitor single cell and network level activity over extended periods of time, without affecting the cell culture under investigation. Since their instrumental role in the 1993 landmark discovery of spontaneous waves in a developing retina, the variety and scope of MEA-based electrophysiology applications have dramatically expanded. Recently, for example, MEA-based electrophysiology culture systems have been used to investigate the suppression of epileptic activity and to study novel plasticity mechanisms in cultured neural networks. Advances in cell culture preparations have led to applications for MEA-based electrophysiology systems in the fields of drug screening, safety pharmacology, stem cell applications and biosensing.
Many conventional MEA systems use printed circuit board (PCB) technology or microfabrication technology or a combination of both techniques to fabricate the electrode array device. The PCB industry has been steadily evolving over the past 40 years. PCB processing is typically performed on large area substrates (e.g., panels that can be 12-inch by 18-inch or larger) outside the cleanroom. Accordingly, PCB processes tend to be more cost-efficient than microfabrication/microelectronic processes, however they produce larger feature sizes when compared to microfabrication processes. That said, complex devices can be created on rigid, flexible, and flex-rigid substrates using PCB technologies.
In general, the limits of PCB technology are characterized by the width of the lines and spaces (L/S) it can produce. These lines and spaces dictate the routing and density of the circuit on any layer of the final construction. Of course multiple layer routing can improve the density of the circuitry constructed, however having multiple layers entails additional fabrication steps and costs. The most common L/S capability of PCB fabrication facilities is 5 microinch or mil (1 mil=25.4 μm). With advanced processing capabilities, some vendors are able to achieve 3 mil L/S. Anything below 3 mils is difficult to achieve in a repeatable and scalable fashion. Some vendors offer 1-2 mil L/S, but this requires an advanced process tool set, which typically results in much higher costs. In order to develop a high-density, high-throughput MEA with single layer processing, a smallest feature size of 1 mil L/S is required. Additionally, transparent substrates (for enabling optical recording and stimulation with inverted microscopy), such as polyethylene terephthalate (PET), are typically uncommon in PCB technology. The typical substrate for PCBs is opaque FR-4 (a glass reinforced epoxy resin) or Kapton (a translucent polyimide), on which copper traces and insulating materials are applied during subsequent processes. PCB technology is a great option for many industrial applications but it has limitations for suitability toward transparent, high-throughput MEAs.
There are numerous methods to fabricate a transparent microelectrode array. A traditional, monolithic approach, typically involves surface micromachining on biocompatible transparent substrates. The microfabrication process starts with the definition of electrically conductive through-holes (called vias) in the substrate that ensure front to back connectivity—the microelectrodes are defined on the front side and the electronics access points are on the backside of the substrate. These through-holes for the vias are typically created using one or more of the following techniques: computer numerically controlled (CNC) micromilling, laser micromachining, injection molding, etching, powder blasting, among others. Conductivity of the via is typically achieved by filling the through-hole with a conductive material, such as a conductive paste using techniques such as screen printing, stencil printing, squeegee printing, inkjet printing and/or metal via electrodeposition, electroless plating, sputtering, or evaporation. Once the vias are sufficiently filled to ensure an ohmic contact between the top and bottom sides, the topside circuitry can be patterned using traditional metallization/metal definition techniques such as metal lift-off, deposit and etch, electroplating utilizing photoresist molds, metal-transfer processes, or embossing. The transparent MEA device is completed by the application of an insulation layer to define the microelectrodes.
Although the use of vias facilitates interconnect paths through (and between) multiple layers of a substrate, fabricating vias is both challenging and expensive, particularly when using transparent substrate materials. For example, fabrication of vias in glass substrates using a powder blasting technique can be challenging to optimize and may result in the weakening of the glass substrate, leading to cracking of the plates during assembly. Fabrication of vias in plastic substrates typically requires micromilling techniques, which tend to be low throughput processes that can become cost-prohibitive when employed in high-volume manufacturing. Injection molding can also be used for fabricating vias, but this process requires a large upfront investment in the micromold master structure. Because vias require strict tolerances on diameter and spacing, any manufacturing process that is employed tends to be expensive, particularly using transparent substrate materials.
The presently disclosed high-throughput microelectrode arrays with flexible circuit array for multilayer interconnect are directed to overcoming one or more of the problems set forth above and/or other problems in the art.